What is a simple game for validation of MCTS?
A good choice might be smaller-scale games of Go, like a 9x9 board. This was the original application domain MCTS was designed for, and the original paper by Brugmann from 1993 details parameters that should lead to an agent that can play above beginner level in what is today a minuscule amount of computational time, in a scaled-down 9x9 grid. 

Go is a good choice for a benchmark because most learning algorithms fail at it pretty badly. The fact that MCTS worked here was a major breakthrough at the time, and helped cement it as a technique for game playing. If your algorithm is not working properly, it is therefore unlikely that it can learn to play Go at the level described in Brugmann's paper.